Conventional post-assembly IC trimming procedures generally end with a locking step, designed to disconnect the trimming block from the package pins, making these pins available for other functions, and disable permanently the trimming block, while preserving the trimmed parameter values status. The post-assembly trimming procedure results in an IC with stable parameters, which cannot be further modified through accidentally trimming. Conventional IC trimming circuitry is described in FIG. 1 and FIG. 2. FIG. 1 shows an IC with a single voltage supply PIN A (VCC) 5, while FIG. 2 shows an IC with two voltage supplies, PIN A (VCC) 5, AND PIN B (LVCC) 6. The energy necessary to change the fuse status is supplied by VCC_Trimming node 8, which is connected either to PIN A 5 or PIN B 6. The programmable fuse (or Zener diodes) array 2 is addressed by the input trimming data 9. This input data includes conventional data to address programmable fuses (or Zener diodes). The output of each individual fuse can be obtained from the fuse status outputs 10 of the programmable fuse array block 2. Each fuse cell 4 typically includes a fuse (or Zener diode), a means to change the fuse status (typically a switch controlled by the input trimming data 9), and a sensing block (not shown) that provides the fuse status output 10.
Conventional techniques for trimming locking is provided in, for example, U.S. Pat. No. 5,079,516 Russell, et al., U.S. Pat. No. 6,472,897 Shyr, et al., and U.S. Pat. No. 6,338,032 Chen. As a general matter, these patents operate by inhibiting the input trimming data transmission from the package pins to the programmable fuse array block 2.
The switch in the fuse cell 4 can include a generic a MOS transistor or a SCR device. Once the fuse is set (blown), the switch cannot be turned on by means of the control terminal; therefore the fuse status cannot be changed once set. The major drawback of this approach is that the fuse (or Zener diode) and the controllable switch in fuse cell 4 remain connected to the package pins after the trimming procedure is completed. A subsequent abnormal voltage (over voltage, high dv/dt voltage spike, electro-static discharge (ESD) spike) on pin A 5 or on pin B 6 may parasitically turn on the switch in fuse cell 4 (for example, by a break-over, snap-back, and/or drain-gate capacitance effect), and accidentally blow out the corresponding fuse (or short-circuit the Zener diode), changing the original trimming procedure results. This means that the above locking methods are not entirely secure against possible environmental voltage conditions.
In another trimming locking approach, described in U.S. Pat. No. 5,079,516 Russell, et al., two fuses have to be blown out, for each trimmed bit. Two SCR devices are also needed for locking each trimmed bit. Thus, this method tends to be complicated and larger chip area is consumed. Moreover, this type of locking procedure based on blown out fuses tends to be only applicable for cavity containing packages, i.e., plastic packages typically cannot use this locking approach.